2-Methyl-3-phenylbutane-2,3-diol, $(\mathrm{CH}{}_3){}_2\mathrm{C}(\mathrm{OH})-\mathrm{C}(\mathrm{OH})(\mathrm{CH}{}_3)(\mathrm{C}{}_6\mathrm{H}{}_5)$, is heated with dilute $\mathrm{H}{}_2\mathrm{SO}{}_4$. The product is

The benzylic tertiary cation $\mathrm{CH}{}_3-\mathrm{C}{}^{+}(\mathrm{Ph}){}^{-}$ is the more stable one. A methyl shift from the neighbouring $\mathrm{C}(\mathrm{CH}{}_3){}_2\mathrm{OH}$ gives $\mathrm{CH}{}_3\mathrm{CO}-\mathrm{C}(\mathrm{CH}{}_3){}_2\mathrm{Ph}$. If the other cation formed, phenyl migration would give the same ketone, so 3-methyl-3-phenylbutan-2-one is obtained either way.